The Air Quality Sensor Performance Evaluation Center (AQ-SPEC) program at the South Coast Air Quality Management District (SCAQMD) in California, USA, is one of the few globally recognized quality test facilities for air quality sensors. The operating principle of the center is simple: it performs highly qualified performance tests for air quality sensors and publishes the results and an evaluation report on their public web site. It has no commercial affiliations and does only technical evaluations, and hence does not issue certificates or approvals for the tested devices.

A brand new evaluation report for the Vaisala AQT410 Air Quality Transmitter by AQ-SPEC has recently been published with excellent performance results. When compared head-on with other electrochemical cell based devices on the list, the AQT410 leads the pack for all the three gases tested (carbon monoxide (CO), ozone (O₃), nitrogen dioxide (NO₂)). Worth noting is that the gas sensing module in AQT420 is identical to the AQT410, so the results apply equally well to AQT420.

The figure of merit typically used in air quality sensor testing is the correlation factor R2 (for the scientifically oriented, the R2 factor is more exactly known as the coefficient of determination) between the readings from the sensor under test and the readings from a certified reference analyzer. A universally accepted scale for sensor rating does not exist, but typically the sensor is considered acceptable when the R2 factor exceeds 0.4, good when it exceeds 0.6 and excellent for correlations over 0.8.

The following table lists the correlation results for five-minute average readings as reported by AQ-SPEC.

Manufacturer(Model)

Type

Pollutant(s)

*Field
R2

Vaisala(AQT410 v.1.15)

Electrochem

CO, NO₂, SO₂ & O₃

CO: R2 ~ 0.80 to 0.83
NO₂: R2 ~ 0.43 to 0.61
O₃: R2 ~ 0.66 to 0.82

Source: http://www.aqmd.gov/aq-spec/evaluations/summary.

For the tricky ozone gas O3, the AQ-SPEC test yields R2 factors between 0.66 and 0.82 for the three AQT410 units tested – an excellent result. For carbon monoxide CO, the AQT410 five-min correlations all surpassed 0.8, and the one hour averages even approached 0.90. For nitrogen dioxide NO2, the 0.43 - 0.61 correlation factors ranged between 0.43 and 0.61 - figures that are indeed quite good but more modest than those often seen for AQT410. To be noted for the NO2 results are the low levels of NO2 in the site during the evaluation period, over the weekends approaching the five ppb detection limit for the device.

When rating an air quality gas sensor device no single gas can be considered as the most important one – the applications and conditions vary and different gases are significant in different situations. Therefore, all three gases have to be considered together, and in this air quality sensor triathlon the AQT410 truly stands out.

Contributor:

Mikko Laakso

Business Development Manager, Air Quality

Mikko has over 20 years of experience in environmental sensors and instrumentation. During his years at Vaisala, he has worked especially with introducing new technologies and instruments to the environmental sensing marketplace. Mikko has hands-on experience from all stages of new product development including sensor research, product concepting and R&D, market assessment and sales ramp-up.

In his current position, Mikko helps customers, such as environmental protection agencies, air quality specialists, environmental consultants and smart city projects, to find new solutions for air quality monitoring and management on a global level. Vaisala’s offering for these customers ranges from compact air quality and weather sensors to full solutions including sensor networks and associated air quality data management and modelling.